This invention relates to humidity sensor assemblies which include a film-type humidity sensing element, and more particularly, to an enclosure for a film-type humidity sensing element.
Film type capacitive humidity sensing elements include a thin planar dielectric film having conductive coatings disposed on its planar surfaces which are the electrodes for the sensing element. Contact portions are formed on the conductive electrode to facilitate connection of the humidity sensing element to the circuit with which it is used. Film-type humidity sensing elements are very delicate structures which are easily damaged.
Known humidity sensing elements are supported on a base or header by a terminal structure which connects the sensing element in the circuit in which it is used. The sensing element is enclosed by a cover which is mounted on the header. Examples of such sensor assemblies are disclosed in U.S. Pat. Nos. 3,914,982 and 4,920,451. In the sensor assembly disclosed in the '451 patent, the humidity sensing element is carried by the terminals and the cover merely encloses the sensing element. In the sensor assembly disclosed in the '982 patent, the humidity sensing element is mounted in a bearer which in turn is mounted on the terminal bearing header. In both arrangements, the lead structure supports the humidity sensing element, and consequently, the potential exists for damage to the humidity sensing element. A further consideration is that, in use, humidity sensor assemblies of this type are generally mounted on a circuit board by their terminal assemblies which increases the possibility of damage to the humidity sensing element of the assembly.
In U.S. Pat. No. 3,168,829 there is disclosed a capacitive film-type humidity sensor which includes a two part casing which encloses the humidity sensing element and secures the element in the casing by trapping its peripheral edge in an annular groove formed by mating surfaces of the casing members. Although this arrangement affords more protection for the sensing element than is provided by other known prior art sensor assemblies, the casing serves as a portion of the terminal structure and thus must be made of an electrically conductive material. This not only precludes the use of an inexpensive plastic material for the casing, but also requires that the casing be part of the electrical circuit in which the sensor assembly is being used. Moreover, the terminal structure is the primary structure for mounting the sensor assembly in the circuit, and one lead is connected to the sensing element at its unsupported center. Thus, the possibility exists that a force applied to this lead could result in damage to the sensing element.